1. Field of the Invention
The present invention relates to ink pens for ink-jet printers, and more particularly, to an apparatus for controlling ink leakage from the reservoir of an ink pen.
2. Description of Related Art
Ink-jet printers have become established as reliable and efficient printing devices. Typically, an ink-jet printer utilizes a print head which is moved relative to a printing surface. A control system activates the moving print head at the appropriate locations causing the print head to eject, or jet, ink drops onto the printing surface to form desired images and characters. Such printers typically include an ink pen which serves as a reservoir for storing ink and provides a means of supplying ink, as needed, to the print head.
There are two commonly used systems for ejecting ink from a print head. The first is a thermal bubble system and the second is a piezoelectric system. A print head using either system typically includes a plurality of orifices, each orifice having an associated chamber. In operation, ink is supplied via an inlet to the chamber. Upon activation, the ink is forced, or jetted, from the chamber through the orifice and onto the printing surface. In thermal bubble type print heads, the ink in the chamber is heated or vaporized, typically by a thin film resistor. The rapid expansion which results from vaporization of the ink forces a quantity of ink from the chamber through the orifice. In piezoelectric type print heads, a piezoelectric element creates a pressure wave within the chamber which ejects a quantity of ink through the orifice.
Although both thermal bubble and piezoelectric print heads provide a reliable and efficient means of jetting ink from an orifice, both types of print heads generally have no mechanism to prevent the free flow of ink through the orifice when the print head is not activated. If this occurs, ink may leak, or drool, uncontrollably through the print head. Typically, printers are provided with catch basins to catch and contain ink dripping from the print head. This helps to prevent the ink from damaging the printer. However, the ink may drip onto the printing surface to produce an undesirable ink spot. In addition, leaking ink may build up on the print head and impair the proper operation of the print head. In any case, a leaking ink pen will usually need to be discarded and replaced.
To alleviate these problems, many ink-jet printers supply ink from the ink pen to the print head at a slight underpressure or backpressure. As used herein a positive backpressure is used to refer to a pressure within an ink pen that is lower than the ambient pressure surrounding the print head orifice.
To be effective, the backpressure must be maintained within a desired operating range. That is, the backpressure must be large enough to prevent the unwanted free flow of ink through the orifice. At the same time, the backpressure must be small enough that the print head, when activated, can overcome the backpressure and eject the ink in a consistent and predictable manner. To meet these constraints and provide optimum operation of the ink-jet printer, a fairly constant and predictable backpressure should be maintained.
The backpressure of an ink pen is affected by changes in either the ambient pressure or the internal pressure. For example, if an ink pen is subject to an increase in altitude, such as during transport aboard an aircraft, the ambient pressure may decrease substantially. Unless the backpressure of the ink pen increases accordingly, the ambient pressure level may drop below that of the backpressure and ink will likely leak from the print head. In addition, as ink is depleted from the ink pen reservoir the backpressure within the ink pen will tend to increase. Without some mechanism to compensate for this, the backpressure may exceed the operating range of the print head and the ink pen will become inoperative. Temperature variations may cause the ink and air within the ink pen to contract or expand, thereby affecting the backpressure. All of these factors must be accounted for in order to ensure consistent trouble-free operation of the ink-jet printer.
One type of ink pen uses an expandable bladder in conjunction with a vent to maintain the proper backpressure within an ink-jet pen. The expandable bladder is situated within the reservoir and configured to expand or contract in response to depletion of ink from the reservoir, pressure changes, temperature variations, or the like. Typically, the bladder is biased with a spring or some similar mechanism which resists expansion of the bladder. This resistance helps to maintain a backpressure within the reservoir.
In conjunction with the expandable bladder, some pens incorporate a vent. The vent is typically configured to selectively allow the entry of atmospheric air into the ink reservoir when the backpressure reaches an undesirable level. The ingress of air through the vent lowers the backpressure. In this manner, the biased expandable bladder serves to create the necessary backpressure and the controlled ingress of air through the vent prevents the backpressure from exceeding the desired range.
The combination of an expandable bladder and a vent has proven to be an efficient and effective mechanism for creating and maintaining the desired backpressure within the reservoir of an ink pen. However, under extreme environmental conditions, or in the case of failure of the expandable bladder or a breach of the integrity of the ink reservoir it is sometimes possible for the backpressure in the ink reservoir to drop below the desired range. In some cases, such conditions may even create a negative backpressure (that is, a pressure within the reservoir that is higher than ambient) within the ink reservoir.
Should this occur, it is possible for ink to be forced from the reservoir. Ink forced from the reservoir will typically exit through either the print head or the vent. As discussed above, printers are typically equipped to minimize damage from ink leaking through the print head. On the other hand, ink leaking through the vent can have disastrous consequences.
In some printer configurations, no catch basin is provided to catch ink leaking from the vent. Moreover, given the usual location of the vent, ink dripping from the vent can land directly on exposed electrical circuits and electrical contacts. If this occurs, the printer may be severely damaged.